#!/usr/bin/perl -w

$|++;
use strict;
use Fcntl;
use POSIX qw(setsid);
my $PROGRAM = 'cleaner.pl';
my $LOCKFILE = "/var/lock/$PROGRAM";

# use a lockfile to prevent spawning duplicate processes
unless ( sysopen(my $fh, $LOCKFILE, O_CREAT | O_EXCL | O_RDWR, 0600) ) {
    close $fh;
    # the lockfile already exists
    print "Lockfile $LOCKFILE aready exists.\nChecking for running $PROGRAM process...";
    # sleep for a couple of seconds to make sure we don't race
    # by this time the process should be in the process table
    sleep 2;
    my @ps = grep{ m/\Q$PROGRAM\E/ } `ps -C $PROGRAM`;
    # we expect 1 copy (us) running
    # print "@ps\n";
    die "\nThere is already a copy of $PROGRAM running!\n" if @ps > 1;
    print "None!\n";
}

# now demonize it
defined(my $pid = fork)   or die "Can't fork: $!";
exit 0 if $pid;

chdir '/'                 or die "Can't chdir to /: $!";
umask 0;
setsid()                  or die "Can't start a new session: $!";

$SIG{INT} = $SIG{TERM} = sub { 
    unlink $LOCKFILE or warn "Could not unlink $LOCKFILE for $PROGRAM\n"; 
    $dbh->disconnect if $dbh;
    exit; 
};
$SIG{HUP} = sub { 
    warn "Caught HUP " . time() . "\n"; 
};

print "Started $0 daemon OK\n\n";

open STDIN, '/dev/null'   or die "Can't read /dev/null: $!";
open STDOUT, '>/dev/null' or die "Can't write to /dev/null: $!";
open STDERR, '>/dev/null' or die "Can't write to /dev/null: $!";

# this is the main loop that runs forever, once every 30 seconds
# we don't want to die so we eval our function in case it chokes
# we have closed STDERR so we won't know if it is choking 
# but we can change the code as required for debugging.
while (1) {
    eval{ blah() };
    sleep 30;
}